1. Field of Invention
The present invention relates to uncooling infrared and terahertz detectors, and more particularly to a microbolometer and method for manufacturing the same.
2. Description of Related Arts
The infrared detector transforms the invisible infrared heat radiation into the detectable electrical signal to achieve the observation to the external affairs. There are two kinds of infrared detectors: the quantum detector and the thermal detector. The thermal detector is also known as the uncooling infrared detector which can be operated at the room temperature, and has some advantages of good stability, high integration and low price, and wide application prospect in the military, commercial and civil fields. The pyroelectric, thermocouple and thermal resistor are three main types of the uncooling infrared detector, wherein the microbolometer focal plane detector based on the thermal resistor is a rapidly developed uncooling infrared detector in recent years (referring to Leonard P. Chen, “Advanced FPAs for Multiple Applications” Proc. SPIE, 4721, 1-15 (2002) literature). The terahertz detector transforms the electromagnetic wave radiation of the terahertz band with the longer wavelength into the detectable electrical signal to achieve the observation to the external affairs. There are various types of terahertz detectors, wherein the uncooling terahertz microbolometer has the similar structure with the uncooling infrared microbolometer, so it can be obtained by improving the latter. The infrared or terahertz radiation detection process of the microbolometer is mainly achieved by the suspended micro-bridge. Therefore, the suspended micro-bridge is the key to affect the manufacture and performance of the device. The microbolometer has some special demands, which are reflected in that the related materials should have proper electrical, optical and mechanical properties, for the film materials of the suspended micro-bridge, especially the core thermistor material.
A lot of materials can be used as the thermistor material of the microbolometer. The vanadium oxide films have excellent electrical and optical properties, and high integration while preparing. Therefore, they are the most commonly used thermistor materials of the high-performance microbolometer and widely used in the uncooling infrared and terahertz detectors. The U.S. Pat. No. 6,489,613, which is applied by Toru Mori and Katsuya Kawano, NEC Company and patented on Dec. 3, 2002, discloses a method of manufacturing the vanadium oxide films for the microbolometer. In this invention, some special metals are doped into the vanadium oxide by sol-gel technology such that the electrical properties of the vanadium oxide meet the demands of the device. Literature H. Jerominek, F. Picard, et al., “Micromachined, uncooling, VO2-based, IR bolometer arrays”, Proc. SPIE, 2746, 60-71 (1996) discloses a micro-bridge structure of the microbolometer based on the vanadium oxide thermistor film. However, the electric structure of the vanadium atom is 3d34s2, wherein 4s and 3d orbits can lose some or all electrons, so the vanadium oxide films prepared by the conventional method have some inherent shortcomings of complex valence of element V in the vanadium oxide films, and poor stability of the chemical structures of the films. For example, while preparing the vanadium oxide films by magnetron sputtering, element V generally has 0, +2, +3, +4 and +5 valences (referring to Xiaomei. Wang, Xiangdong. Xu, et al., “Controlling the growth of VOx films for various optoelectronic applications”, Proceedings of the 2009 16th IEEE International Symposium on the Physical and Failure Analysis of Integrated Circuits, IPFA, p 572-576 (2009) literature). Since element V has the complex composition, slight fluctuation in the preparation process might greatly impact on the chemical compositions of the vanadium oxide films, so that the electrical, optical and mechanical properties of the films are significantly changed, thereby affecting the performance of the device. Therefore, the main shortcomings of the microbolometer based on the vanadium oxide films are that the preparation is difficult, and the repeatability and stability of the product are poor.
On the other hand, carbon nanotubes are very important one-dimensional nanomaterials. Since 1991, Japanese Iijima found the carbon nanotubes (referring to Sumio Iijima, “Helical microtubules of graphitic carbon”, Nature, 354, 56, (1991) literature), more and more researches have shown that these special one-dimensional nanomaterials have some unique physical and chemical properties, and broad application prospects. Firstly, carbon nanotubes have excellent chemical stability. In the vacuum condition, carbon nanotubes can maintain the stable chemical structures at 1200° C. Furthermore, in the atmospheric environment, carbon nanotubes can maintain the stable chemical structures under 650° C. Obviously, the chemical stability of the carbon nanotubes is higher than that of the vanadium oxide films. In addition, carbon nanotubes have excellent electrical and optical properties. For example, it is reported that the temperature coefficient of resistance (TCR) of carbon nanotubes can reach 0.3-2.5%/K, and the optical absorption coefficient of carbon nanotubes can reach 104-5 cm−1 under certain conditions (referring to M. E. Itkis, F. Borondics, A. Yu, R. C. Haddon, “Bolometric Infrared Photoresponse of Suspended Single-Walled Carbon Nanotube Films”, Science, 312, 413-416 (2006) literature). Therefore, carbon nanotubes hold great potential for acting as thermistor materials, and are capable of overcoming the shortcoming of poor chemical stability of the conventional heat sensitive films, such as vanadium oxide and etc.
Currently, a variety of tries are made in the application field of the carbon nanotube device. U.S. Pat. No. 7,057,402B2 applied by Barrett E. Cole, et al., Honeywell Company and patented on Jun. 6, 2006 discloses a sensor based on the carbon-nanotubes. This invention improves the absorbing properties of the material to infrared light by catalyst-inducing the growth of upwardly standing carbon nanotube bundles, so that the temperature of the system is increased under the action of infrared light, the corresponding temperature changes are measured by the Pt sensor under the carbon nanotubes. In this invention, the standing carbon-nanotubes play a major role in absorbing the infrared light.
Chinese patent ZL 02114434.6 applied by Junhua Liu, et al., and patented on Jan. 26, 2005 discloses a carbon nanotube film micromachined infrared detector. This invention takes the carbon nanotubes as the absorbing material for infrared radiation. By measuring the changes of the thermal warpage or the natural oscillation frequency of the micromachined infrared detector which are caused by the carbon nanotubes, the detection of the radiation intensity is achieved. Chinese patent ZL 200320109976.X applied by Junhua Liu, et al., and patented on May 4, 2005 discloses a carbon nanotube film piezoresistive heat-sensitive infrared detector. Also, this invention takes the carbon nanotubes as the infrared light absorbing and heat sensitive materials. When the micromachine is optically radiated, the thermal warpage or the resonance frequency drift will appear, so that the piezoresistive factor of the carbon nanotubes on the micromachine is changed. Using the piezoresistive effect of the carbon nanotubes, the temperature distribution is sensed by the drift of the resistance change frequency of the carbon nanotubes, thereby measuring the radiation intensity. This invention resolves the low sensitivity and high cost of the pyroelectric detector. However, the carbon nanotubes, especially the carbon nanotubes reclining on the surface of the substrate have non-ideal infrared absorbing performance (referring to Z. Wu, Z. Chen, et al., “Transparent, Conductive Carbon Nanotube Films”, Science, 305, 1273-1276 (2004)). This shortcoming will seriously impact on the performance of related device.
Recent studies show that constructing the carbon nanotubes in the suspended structure can obviously improve the heat sensitive property of the carbon nanotubes, which can meet the demands of infrared detection under certain conditions (referring to M. E. Itkis, F. Borondics, A. Yu, R. C. Haddon, “Bolometric Infrared Photoresponse of Suspended Single-Walled Carbon Nanotube Films”, Science, 312, 413-416 (2006) literature). However, this structure based on the single carbon nanotubes has an obvious shortcoming that, π electrons of the carbon nanotubes have stronger conductivity, and consequently, direct application of the single carbon nanotubes as the heat sensitive material will lead to a very small resistance of the film. Therefore, the slight change of the carbon nanotube resistance caused by infrared radiation only can be detected in the ultra-low temperature of liquid helium condition, which shows that the micro-bridge structure based on the single carbon nanotubes can not meet the demands of the uncooling infrared detector, and more particularly the uncooling terahertz detector with higher sensitivity requirement. The conventional method for increasing the carbon nanotube resistance is as below. Disperse the carbon nanotubes in some polymer systems, such as polycarbonate and polystyrene to form the carbon nanotube-polymer composite films (referring to A. E. Aliev, “Bolometric detector on the basis of single-wall carbon nanotube/polymer composite”, Infrared Physics & Technology, 51, 541-545 (2008) literature). Although these carbon nanotube-polymer composite films can meet the demand for the electrical properties of the microbolometer, they have another shortcoming, namely, in the wavelength of 1-10 μm, the absorbing capability of the carbon nanotubes to infrared light is poor (absorbing coefficient is small), the absorbance is only 10% (referring to Z. Wu, Z. Chen, et al., “Transparent, Conductive Carbon Nanotube Films”, Science, 305, 1273-1276 (2004)). Common polymer materials are not helpful for improving the optical properties of the carbon nanotubes. Therefore, it is difficult for the common composite materials of carbon nanotube and polymer to meet the demands of the infrared or terahertz detectors for light absorption performance.
In short, the conductivity and chemical stability of the vanadium oxide films have some deficiencies and need to be improved. Also, the carbon nanotubes have some deficiencies in the electrical and optical properties. Therefore, the single carbon nanotubes or common carbon nanotube-polymer composite films are not suitable for direct application in the uncooling infrared or terahertz detectors to act as the infrared or terahertz light absorbing and thermistor materials.